\subsubsection{Overhead of writing to witness nodes}
\label{sec:eval-barrier}
\begin{figure}[t]
\centerline{\includegraphics[angle=0, width=1\textwidth]{graphs/salus_new_active.pdf}}
\caption{\label{graph:new_active} Overhead of storing metadata on additional witness nodes.
The ``New'' protocol stores certificates on witness nodes while the ``Old'' protocol
does not.}
\end{figure}

To measure the overhead of \sys' new active storage protocol, which stores certificates
on additional witness nodes, we compare the throughput of the new protocol to
that of our earlier active storage protocol~\cite{yang13salus}, which stores certificates on only three \Dn{s}.
We perform this set of experiments on 12 servers, each equipped with 16 cores, 64GB of
memory, and a single disk, and increase the number of clients until the system
is saturated. Since witness nodes are only accessed for writes,
we only measure write throughputs in these experiments.

Figure~\ref{graph:new_active} shows that the aggregate throughput of the new protocol
is comparable to that of the old protocol under both the sequential and the random write
workloads. This result
once again confirms one of the basic ideas of this dissertation: for storage systems
processing large bulk of data, adding a small metadata can enhance the
robustness of the system with little overhead.
